Heretofore, highly-elastic rubber is used for the carcass part and the like of a tire. As a means of enhancing the elasticity of rubber, there is disclosed a method of increasing the amount of the filler such as carbon black or the like, for example, in Patent Reference 1, and there is also known a method of increasing the amount of sulfur in the vulcanizing agent to thereby increase the crosslinking point, etc.; however, the case of increasing the amount of the filler such as carbon black or the like is problematic in that the factory workability of the rubber composition as well as the resistance to fracture such as the elongation at breakage or the like thereof may worsen and the heat generation of the rubber composition may increase. In addition, the case of increasing the amount of sulfur in the vulcanizing agent is also problematic in that the elongation at breakage may lower and the physical change owing to thermal degradation may increase.
In this connection, as a means of enhancing the elasticity of a rubber composition with suppressing the reduction in the elongation at breakage thereof, there is disclosed a method of adding thereto an unmodified novolak-type phenolic resin to be obtained through condensation of a phenol and an aldehyde in the presence of an acid catalyst, or a modified novolak-type phenolic resin that has been modified with an unsaturated oil such as tall oil, cashew oil or the like or with an aromatic hydrocarbon such as xylene, mesitylene or the like, and hexamethylenetetramine as a curing agent capable of curing the resin, for example, in Patent References 2 and 3.
Patent Reference 4 proposes a method of using a resol-type phenolic resin in place of the above-mentioned hexamethylenetetramine as the curing agent for the novolak-type phenolic resin to be added to rubber for the purpose of improving the processing safety of the rubber composition.
Patent Reference 1: JP-A 9-272307
Patent Reference 2: JP-A 5-98081
Patent Reference 3: JP-A 2001-226528
Patent Reference 4: JP-A 57-57734